1. Field of the Invention
The present invention relates to an insulating film evaluation method of evaluating an insulating film used in a semiconductor device.
2. Description of the Related Art
With a conventional method of evaluating an insulating film used in a semiconductor device, there is obtained the amount of charge injected until dielectric breakdown of the insulating film occurs. The amount of injected charge can be obtained from the sum total of the products of (1) the values of currents flowing to the insulating film until the dielectric breakdown occurs and (2) the time periods during which the currents flow thereto. This method necessitates measuring each current flow time period until dielectric breakdown occurs. When performing this time measurement, it is important to judge whether there occurs dielectric breakdown of the insulating film.
Once dielectric breakdown of the insulating film occurs, the resistance of the insulating film is decreased, so that a voltage generated after the dielectric breakdown by the application of a current to the insulating film becomes lower than a voltage generated before the dielectric breakdown. To judge using a computer whether there occurs dielectric breakdown of the insulating film after the application of the current by utilizing this physical phenomenon, there is used a method with which a voltage Vi generated in the insulating film by the application of a current Ii to the insulating film is measured and it is judged that dielectric breakdown occurs if the voltage Vi is lower than a reference voltage Va (having a constant value). For instance, this reference voltage Va used to make the dielectric breakdown judgment is set with reference to an initial voltage V0 measured by applying an initial current 10 to the insulating film for which current application is not yet performed. Alternatively, the reference voltage Va is determined according to experimental data.
In the case where the current Ii applied to the insulating film is constant, the voltage Vi measured after dielectric breakdown becomes lower than the voltage Vi measured before the dielectric breakdown. This means that it is possible for a computer to make the dielectric breakdown judgment without any problems using the conventional judgment method if the judgment reference voltage Va is lower than the initial voltage V0 and is also higher than a voltage that is estimated from a resistance resulting from dielectric breakdown. However, in the case where the current Ii applied to the insulating film is not constant and is continuously or discontinuously increased during the measurement, there may be cases where the voltage Vi measured after the dielectric breakdown becomes higher than the initial voltage V0. Consequently, there may be cases where the judgment of dielectric breakdown cannot be properly made using the foregoing method with which it is judged that there occurs dielectric breakdown if the measured voltage Vi drops below the judgment reference voltage Va ( less than V0), that is, if a condition of xe2x80x9cVa greater than Vixe2x80x9d is satisfied.
The present invention has been made in the light of the above-mentioned problem and an object of the present invention is to provide an insulating film evaluation method with which it is properly judged whether there occurs dielectric breakdown of an insulating film even if the current Ii applied to the insulating film is not constant and is continuously or discontinuously increased during measurement.
To achieve the stated object, with an insulating film evaluation method of the present invention, it is judged whether there occurs dielectric breakdown of an insulating film by comparing an absolute value |Vi| of the ith measured voltage Vi with an absolute value |Vi+1| of the i+1th measured voltage Vi+1.
With this method of the present invention, it is possible to properly judge whether there occurs dielectric breakdown of an insulating film not only in the case where a current Ii flowing to the insulating film is constant but also in the case where the current Ii is continuously or discontinuously increased.